1. Field of the Invention
The present invention relates generally to the use of multispectral interferometry for measuring the thickness of, or for the thickness mapping of substrates such as semiconductor wafers, photo-masks, or the like.
2. Prior Art
Capacitance gauges by ADE Corp. are currently used to scan over a semiconductor wafer to generate a thickness map. Although accurate to 0.1 microns they suffer from poor spatial resolution due to the size of the probe.
Oblique incidence optical interferometry at a single wavelength has been used by GCA-Tropel to measure large path differences. While this method could be adapted in accordance with this invention to make measurements it uses a large prism whose size and thickness becomes very large as the semiconductor wafer diameter increases. The oblique incidence system can create a thickness map of a highly distorted surface.
The GCA/Tropel Autosort.TM. system consists of a grazing incidence Fizeau interferometer illuminated by a He-Ne laser at 0.6328.mu..
Interferemetric measurement of optical surfaces using a single wavelength Fizeau Interferometer results in ambiguities if the surface deviates from a plane by more than half a wave. This limitation has been minimized in the GCA/Tropel Autosort.TM. system by making single wavelength measurements at grazing incidence to reduce the optical path in the medium separating the two surfaces.
A second method of expanding the thickness range has been implemented in a Hughes full wafer mapper PD9103 1 which uses multi-spectral imaging at normal incidence together with pattern matching of reflectance spectra to determine film thickness' up to 8.mu. of silicon, 20.mu. of SiO2 and 30.mu. of air. This instrument is available from Hughes Danbury Optical Systems, Inc., Danbury, Conn. and is similar to the apparatus described in U.S. Pat. No. 5,333,049 to A. M. Ledger, the inventor herein.
In order to prepare thickness maps of a substrate such as silicon wafer it is desirable to unambiguously measure large thickness variations using a visible imaging system. For example AccuFlat.TM. wafers require that a thickness map of a silicon wafer, be measured to 0.05.mu. (relative not absolute) and that this map should be accurate out to the edge of the wafer with a spatial resolution which matches that used by the PACE wafer flattening system. The PACE wafer flattening system does not form part of this invention. It comprises a plasma assisted chemical etching technique which is the subject of numerous patents assigned to Hughes Aircraft Company, the assignee of the instant invention. Currently wafers are measured before and after polishing using a capacitance technique which does not give very accurate results out to the edge of the wafer. Typically a grid of 64.times.64 or 126.times.126 points are needed over a 200 mm diameter wafer to assure correct polishing, which defines the spatial frequency needed.